Experts now believe the ISS issue may be down to outgassing from the visiting Dragon capsule.

Antonius de Rooij, author of the Space Materials Database, told Wired he believes the paint is the probable problem.

He said the size of the capsules, could be key to the problem.

Allowed to be tested, according to this presentation, up to 21 times the amount of contamination on one sensor.

SpaceX says NASA pre-approved all the materials used in the Dragon, and said it is working with a new variant of paint.

WHAT IS THE SAGE SENSOR?

The Stratospheric Aerosol and Gas Experiment III-ISS (SAGE III-ISS) measures Earth's sunscreen, or ozone, along with other gases and aerosols, or tiny particles in the atmosphere.

SAGE III-ISS makes its measurements by locking onto the sun or moon and scanning the limb, or thin profile of the atmosphere.

The Stratospheric Aerosol and Gas Experiment III-ISS

The instrument makes these measurements from the unique vantage point of the International Space Station (ISS), which increases the scientific value of SAGE III-ISS observations.

The SAGE III sensor launched in February 2017

'SpaceX has scrutinized all external material selections on Dragon and is working with suppliers to develop the molecular deposition rate,' it said.

The white paint is made by Alion, and was developed for NASA and SpaceX.

'For decades,' said Michael Kenny, senior chemist with Alion, said when it was first used.

From the space assets at risk from the capsule's outgassing is the U.S. Laboratory Science Window, a porthole for astronauts and instruments. Pictured, Jeff Williams, Expedition 13 Science Officer, at the U.S. Laboratory Science Window on the ISS.

Dragonfly dragon cargo capsules – and it is believed to have been to be blame.

As space missions evolved, there was a build up on the exteriors of spacecraft, or there could be damage to the spacecraft's electronics.

'Alion' s research led to the development of thermal controls. The outcome of this research was Alion's proprietary Z-93C55 coating. '

However, if Alion Z-93 gets contaminated after curing, it's prone to degradation when UV rays hit it, a phenomenon first documented in 1971.